Streamline definition of LabelMeta aliases

[LandingEllipse]

Some label attributes have multiple monikers associated to provide convenience aliases. For instance, in the PanCam meta map, both sub_instrument and camera point to .//psa:Sub-Instrument/psa:identifier. Currently, this results multiple KV pairs, which is a bit messy, and which anecdotally increases the threshold for defining aliases.

We could alternatively allow keys to be string tuples.
Pros:

• Only one entry per label attribute; less room for mistakes
• Cleaner communication of the availability of aliases; potentially easier to integrate with documentation

Cons:

• Unclear what order the aliases should be in (though attribute name first seems sensible)
• Loss of static key collision warnings (although this could be covered by a test)
• Need to unpack the source meta map dict at runtime anyway

[LandingEllipse]

Alternatively, if we're open to eating the loss of static checking and unpacking cons above, then we could go further: invert the map to deduplicate paths and play on the underlying structure. e.g.:

{
    "emrsp_rm:Mission": {
        "emrsp_rm:mars_sol": ("mars_sol", "sol"),
        "emrsp_rm:Experiment_Cycle": {
            "emrsp_rm:ec_number": ("ec_number", "ec_num"),
            "emrsp_rm:ec_phase": "ec_phase",
        },
        "emrsp_rm:vertical_survey_number": "vs_num",

    },
}